SharePoint sub sites are a great way to segregate both information and security. A sub site is created from a parent site in a tree like structure as required.
Sub sites can use a number of pre-existing templates as a base or even a custom one that you have created. For information on creating a custom template see my previous video on the subject:
A sub site can also have unique permissions if required but by default they will inherit from their parent.
Sub sites in SharePoint are a great way to structure your data and security to needs of your organisation.